Tag Archives: PTR

This is a very popular mistake admins make – it’s missing . i.e dot in the end of hostname. This causes serious problems (and lot of frustration!).

E.g taking example of popular Google’s cname record ghs.google.com. As we know if one would like to use mail.domain.com., he has to point the CNAME record to “ghs.google.com“. Now here if one misses dot in the end of ghs.google.com. – it will give a real value like:

mail.domain.com cname to ghs.google.com.domain.com – thus adding a domain name itself (many DNS control panels do take care of this issue, but quite a lot of them don’t).

So why does that happens?

To understand that, one has to remember that DNS is on a hierarchy based model with . (dot) as top level domain with other TLD’s like com, net, org etc below it i.e

So esstentially it’s much like pealing off. So for www.google.com, it’s like:

And thus if there’s no dot in end, it is (many times) assumed that the specified record value (say ghs.google.com) is a sub value under main zone and therefore the domain name itself is added making actual string like ghs.google.com.domain.com. and eventually nothing works!

And we see admins making faces like:

This not only applies to forward DNS, but reverse DNS too.

Here’s an interesting case of awfully crappy reverse DNS setup of domain name of popular Indian ISP – bharti Airtel. Domain name – airtel.in

now, since admin missed a dot in the end of www.airtelworld.com PTR value, it resulted in addition of main zone which was 17.19.125.in-addr.arpa. in the end which turns out to be www.airtelworld.com.17.19.125.in-addr.arpa.

Completly incorrect and bad setup.

With hope that you won’t miss dot in the end of your hostnames, time for me to get back on cramming for next DDC exam! 🙂